Hello perlisok, and welcome to the Monastery!

In these lines:

    my $fields;
    eval $_;
    die if $@;

    my ( $name, $location, $mapref, $type, $description ) = @$fields;
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you create an empty variable $fields, then you set the values of $name, $location, etc., in the eval, but then you overwrite them in the very next line. This treats $fields as an array reference, but since $fields is still empty, dereferencing it as an array produces an empty list, and the 5 variables lose the values they were assigned in the eval!

Here is the skeleton of a solution:

#! perl
use strict;
use warnings;
use Data::Dumper;

$Data::Dumper::Indent = 0;
$Data::Dumper::Useqq  = 1;
$Data::Dumper::Purity = 1;

my ($name, $location, $siteMapRef, $type, $description, $mapRef);

while (<DATA>)
{
    eval $_;
    die if $@;
    my $fields = [ $name, $location, $siteMapRef, $type, $description 
+];
    $_ = Data::Dumper->new( [ $fields ], [ 'fields' ] )->Dump();
    $_ .= "\n";
}
continue
{
    print $_;
}

__DATA__
$name="MyName";$location="MyLoc";$mapRef="MyRef";$type="MyType";$descr
+iption="MyDesc";
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which produces the output:

$fields = ["MyName","MyLoc",undef,"MyType","MyDesc"];
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Update: If you’d had use strict at the start of your script, it would have caught the problem for you. Always begin every script with:

use strict;
use warnings;
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See The strictures, according to Seuss. :-)

Plus minor edits.

Hope that helps,

I'm not entirely sure what you mean by "It gives no effects after: ...". For instance, is $tempFile not created? If it is, then that would be an effect. Perhaps you mean $megalithFile is empty after the script completes; although, that seems unlikely unless you received some error message (which I'd also class as an effect).

NOTE: I have the first edition of "Programming the Perl DBI" (February 2000), so what I'm looking at may differ from your copy if you have a more recent edition. The while loop you show is on page 24 of my copy - you may be able to use that as a reference point.

The purpose of the examples you're referring to is to show how you could use Data::Dumper to format input records so that they may be evaled. On page 23, you'll see:

my $dumpedValues = $dumper->Dump();
...
eval $dumpedValues;
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From your code above and your description of xxx.txt, you appear to be evaling the single record:

$name="MyName";$location="MyLoc";$mapRef="MyRef";$type="MyType";$descr
+iption="MyDesc";
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In order for

my ( $name, $location, $mapref, $type, $description ) = @$fields;
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to be doing something useful, you should be evaling something that looks more like:

$fields = [ qw{name location mapref type description} ];
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You appear to have removed all the comments that appear in the book. The comment regarding the eval should have provided a clue. My copy has:

### Evaluate perl record string to set $fields array reference
my $fields;
eval $_;
die if $@;
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Two paragraphs before the while loop code, my copy has:

"... use Data::Dumper to format the records ...

I suggest you try doing that.

If you're still having problems, please show your input and output within <code>...</code> tags (as opposed to a prosaic description).